If anyone is curious how the code actually works, I've commented up a version below:

    <script type="text/javascript">

    function matrix() {
        // initialize variables
        for (s = window.screen,
             w = q.width = s.width, // on my monitor: 1920
             h = q.height = s.height,  // on my monitor: 1200
             m = Math.random,   // random number from 0-1
             p = [],
             i = 0; 

             // i ranges from 0 to 255, one element for each character horizontally
             // this is enough characters to fill the entire screen horizontally
             // canvas won't let you draw off the screen - so I could set this to 1000
             i < 256;

             // initialize p (the y coordinate of each character) to start at 1
             p[i++] = 1);

        setInterval(
            // every time we call this function we draw the entire screen a very faint black (with a high transparency of 0.05)
            // this means every 33 milliseconds the screen is getting slightly darker
            // this also acts to darken and fade the green characters - when they are first printed they are dark green, then they slowly fade to black
            function() {
                // draw black (0,0,0) with alpha (transparency) value 0.05
                q.getContext('2d').fillStyle='rgba(0,0,0,0.05)';
                // fill the entire screen
                q.getContext('2d').fillRect(0,0,w,h);
                // #0f0 is a short form for color green (#00FF00)
                q.getContext('2d').fillStyle='#0F0';

                p.map(
                    // this function will be called 256 times - once for each element of array p, 
                    function(v,i){
                    // map over the array p
                    //      v is the value in the array p, which represents the y-coordinate of the text going down
                    //      i is the index of the array p, which represents the x coordinate
                    // start from unicode char code 30,000 (0x7530) then add a random number from 0-33
                    // from wikipedia: http://en.wikipedia.org/wiki/List_of_CJK_Unified_Ideographs,_part_2_of_4
                    //      U+753x 	田 	由 	甲 	申 	甴 	电 	甶 	男 	甸 	甹 	町 	画 	甼 	甽 	甾 	甿
                    //      U+754x 	畀 	畁 	畂 	畃 	畄 	畅 	畆 	畇 	畈 	畉 	畊 	畋 	界 	畍 	畎 	畏
                    //      U+755x 	畐
                    randomNum = m()*33;
                    // note how the asian characters are slightly different shades
                    // of green, this depends on their line thickness etc, and doesn't
                    // really happen for english characters
                    randomAsianChar = String.fromCharCode(30000 + randomNum);

                    q.getContext('2d').fillText(
                        randomAsianChar, 
                        i*10,   // x coordinate - each character is 10 x 10
                        v       // y coordinate 
                    );
                    // draw at least 758 characters down before reseting to the start
                    minimumHeight=758
                    num = minimumHeight+m()*10000;
                    p[i] = (v>num) ? 0 : v+10   // increment the y coordinate by one character (10 pixels), reset when y-coord gets too big
                    })
            },
            33) // call every 33 milliseconds
    }
    </script>

    <body style=margin:0 onload="matrix()"><canvas id=q>